1. Field of the Invention
The present invention relates to an electronic device which constitutes a power supply circuit such as, for example, a head end as a transmitter in a transmission base of a CATV (cable television) system.
2. Description of the Prior Art
In a CATV system, using an ordinary ground wave, a television signal or a satellite broadcasting signal is transmitted to each subscriber of the CATV through a dedicated cable. Then, a head end receives such television signal or satellite broadcasting signal, converts the frequency thereof into a frequency of an intermediate frequency band, and sends out the thus frequency-converted signal again after frequency conversion to the frequency of a channel allocated to the CATV system.
FIG. 7 is a power supply circuit diagram of an electronic device used in such a conventional head end or the like. As shown in the same figure, a power supply block 30 comprises electric components such as a transformer and an electrolytic capacitor (neither shown). A so-called commercial voltage (for example, an AC voltage of 100V or 240V) is fed to an input terminal 31. The thus-inputted commercial voltage is converted to a predetermined DC voltage (for example, a DC voltage of +5V or +9V) by the power-supply block 30 and this converted voltage is fed to an output terminal 32. This output DC voltage serves to drive a frequency converter (not shown), etc.
To the input terminal 31 are connected two protective elements 33, 33 in series between power supply lines to protect the power supply block 30 from a surge voltage such as a thunder voltage for example. A connection point 33a between the protective elements 33, 33 is earthed. By this earthing, (i.e. grounding) when a high voltage such as a thunder voltage is applied induced in the power supply lines, it is bypassed to the earth through the protective elements 33, whereby the power supply block 30 is protected from the high voltage.
However, as to the power supply circuit in the electronic device referred to above, it is prescribed that the power supply circuit should be subjected to a high withstand voltage test in which a high voltage (for example, an AC voltage of 2400V) similar to such a surge voltage as a thunder voltage should be applied as a test voltage to the power supply circuit (according to a safety standard such as UL Standard established by Underwriting Laboratory).
FIG. 8 is an explanatory diagram of the power supply circuit to be subjected to such a high withstand voltage test. As shown in the same figure, one end portion of a switch element 34 is connected to the connection point 33a between the protective elements 33, 33, that is, arresters in the power supply circuit, while the opposite end portion of the switch element 34 is earthed.
To the input terminal 31 is connected a high voltage generator 35.
In this high withstand voltage test, a high voltage (for example an AC voltage of 2400V) generated from the high voltage generator 35 is applied one to two seconds to the input terminal 31 while the switch element 34 is kept OFF, and it is tested whether the power supply block 30 can withstand this high voltage.
This high voltage withstand test is conducted only once in the power supply circuit manufacturing process prior to shipment of product (electronic device). After the test, the switch element 34 is turned ON, and during shipment of the product, the connection point 33a between the protective elements 33, 33 is earthed (ON of the switch element 34).
FIG. 9 is a sectional view of a principal portion of a conventional electronic device C. A chassis 20 is in the shape of a rectangular box formed of a metallic material. The box-shaped chassis 20 comprises four side walls 20a, a lower bottom wall 26b and an upper opening portion 20c. A plurality of cylindrical holding members 21 formed of a metallic material are each centrally provided with a hole 21a and are secured to the bottom wall 26b.
A printed circuit board 22 formed of, for example, a resin material or a molding material has circular holes 22e in a plurality of predetermined positions, for example, in the four corners. On the upper surface of the printed circuit board 22 are formed desired circuit patterns, including earth patterns 22d, with use of copper foil for example. At predetermined positions of the circuit patterns 22a are arranged and connected electric components of a power supply block 30, such as a power transformer 22c, as well as an electrolytic capacitor and a diode (neither shown).
Also disposed and connected is a protective element 22b such as an arrester to protect the power supply block from a surge voltage.
One terminal 22f of the protective element 22b is connected to an earth pattern 22d.
Thus, on the printed circuit board 22 are arranged and connected the protective element 22b and electric components, including the power transformer 22c, which constitute such a power supply circuit as shown in FIG. 7.
Screws 23 are used, which are round head or pan head machine screws formed of a metallic material. The printed circuit board 22 is disposed within the chassis 20, and the screws 23 are threaded and fixed into the holes 22e of the printed circuit board 22 located on the holding members 21.
The earth pattern located at one end portion of the printed circuit board 22 and the associated side wall 20a of the chassis 20 are connected together with solder 25. By this connection with solder 25 one terminal 22f of the protective element 22b such as an arrester is earthed to the chassis 20.
Between the earth pattern 22d with one terminal 22f of the protective element 22b connected thereto and the chassis 20 there is constituted the switch element 34 shown in FIG. 8. The state of connection with solder 25 and that of non-connection (separation) correspond respectively to ON and OFF states of the switch element 34 in the foregoing high withstand voltage test.
An upper cover 24, which is formed by a metallic plate, comprises a rectangular upper wall 24a and a plurality of resilient mounting portions 24b bent from end portions of four sides of the upper wall 24a and extending downward.
The upper cover 24 is mounted in such a manner that its upper wall 24a covers the upper opening portion 20c of the chassis 20 and its mounting portions 24b grip the side walls 20a of the chassis.
The following description is now provided about a high withstand voltage test for the electronic device C constructed as above.
First, the printed circuit board 22 is screwed into the chassis 20. At this time, soldering for connecting an earth pattern 22d on the board 22 to the side wall 20a of the chassis 20 is not performed at the solder 25, and one terminal 22f of the protective element 22b is not earthed to the chassis 20, that is, the switch element 34 is turned OFF.
Next, the upper cover 24 is applied to the chassis 20 in the same form as that of a completed product.
In this form, a high voltage is applied from the high voltage generator 35 to the input terminal 31 of the power supply circuit shown in FIG. 8 and which constitutes the electronic device C, and there is conducted a high withstand voltage test for the power supply block 30. That is, during this application of the high voltage, the power supply block 30 is not protected from a surge voltage by the protective element 22b.
Then, the upper cover 24 of the electronic device C having gone through the process of the above high withstand voltage test is removed and the earth pattern 22d on the printed circuit board 22 and the side wall 20a of the chassis 20 are soldered using the solder 25, that is, the switch element 34 is turned ON. As a result of this soldering, one terminal 22f of the protective element 22b is electrically connected to the chassis 20, and the protective element 22b comes to exhibit the function of protecting the power supply block 30.
The power supply block 30 can now be protected from a surge voltage by the protective element 22b.
Next, the upper cover 24 is applied to the chassis 20 to complete the electronic device C having undergone the high withstand voltage test.
However, in the high withstand voltage test for the conventional electronic device during assembly thereof, as mentioned above, with the earth-side terminal of the protective element such as an arrester not soldered (earthed) to the chassis, the upper cover is applied to the chassis and a high voltage is applied to the electronic device. Then, the upper cover of the electronic device after the test in question is removed and the earth pattern with the earth-side terminal of the protective element connected thereto is soldered (earthed) to the chassis.
Thus, for the conventional electronic device, the high withstand voltage test is conducted with the upper cover applied to the chassis, thereafter, the upper cover is removed and the soldering work for earthing the protecting element is performed, and after the soldering work, the upper cover is again applied to the chassis. This process is complicated and troublesome, making the working efficiency poor.